The Guardian UK sifted out the top 10 Agatha Christie novels back in 2009, in chronological order:
The Murder of Roger Ackroyd (1926) H
Peril at End House (1932) H
Murder on the Orient Express (1934) H
The ABC Murders (1935) H
And Then There Were None (1939)
Five Little Pigs (1943) H
Crooked House (1949)
A Murder is Announced (1950) M
Endless Night (1967)
Curtain: Poirot’s Last case (1975) H
Six of these are Hercule Poirot mysteries. Christie brought Hercule Poirot to life in 1916, when she was inspired to write her first crime novel, The Mysterious Affair at Styles. She was unable to account for the creation of this extraordinary little being, with his fanatical love of order, his delicious conceit, his sexless cosmopolitan charm. In her autobiography she referred, almost cursorily, to how Poirot was inspired by the wartime Belgian refugees who had lived in her home town of Torquay; but this was no explanation of the mystery of creativity, the instinct that had guided her so surely.
Poirot’s intelligence and quirkiness are what appeal to me.He is incomparably fussy about his appearance; and he is extremely confident and competent in his deductions. On occasions he has warned someone ahead of time not to follow through a plan or scheme or relationship. Although one might have seen all that he has seen up to that moment, what is obvious to him becomes merely foreshadowing for the reader.